1. Field of the Invention
The present invention relates to an elastic wave device using a trapping energy effect, and particularly relates to a technique to miniaturize the elastic wave device.
2. Description of the Related Art
When controlling frequency or selecting frequency in a small-sized high frequency communication device such as a portable terminal, there is a is small-sized high frequency communication device employing an oscillator or a filter using a trapping energy type resonator (hereinafter, the oscillator and the filter are generically called an elastic wave device).
A trapping energy type elastic wave device obtains a necessary frequency response by using resonance to occur by trapping energy of thickness vibration in a piezoelectric body of quartz-crystal or the like. The thickness vibration to be used in the elastic wave device of this type vibrates at a frequency on a high frequency side in the vicinity of a cutoff frequency of the piezoelectric body. Thus, electrodes for excitation are provided at center portions on an upper surface and a lower surface of, for example, a plate-shaped piezoelectric body, and a resonant frequency of thickness vibration in a region having the electrodes provided thereon is reduced by a mass loading effect of the electrodes to be lower than a cutoff frequency of the thickness vibration in a peripheral portion to thereby achieve trapping of the energy of the thickness vibration in the region having the electrodes provided thereon.
Here, a cutoff frequency of a piezoelectric body is in inverse proportion to a thickness of the piezoelectric body, so that it is necessary to thin the piezoelectric body in order to increase a resonant frequency of vibration to be obtained by trapping energy, (which is called a trapping energy mode), and in combination with miniaturization and sophistication in performance of a portable terminal, and the like, the elastic wave device is rapidly miniaturized.
However, when the piezoelectric body is thinned in order to increase the resonant frequency, it is also necessary to thin the electrodes to be suitable for trapping energy. When the electrodes are thinned, there is a fear that an insertion loss of the whole elastic wave device is increased due to an insufficiency of electric conductivity.
Normally, however, there is a high fear that masses of the electrodes constituted by metal and the like change with time due to oxidation, corrosion, or the like, and the elastic wave device to have the electrodes directly provided on the region where trapping energy is performed, (which will be called a trapping energy portion, hereinafter), has a problem that the resonant frequency changes by an influence of a mass change in the electrodes.
Thus, in order to suppress the above influence of the change with time in the electrode masses, various types of elastic wave devices to have no electrode provided on a region where resonance is generated are proposed. In Patent Document 1, for example, there is disclosed a resonator in which electrodes for excitation are provided at both right and left ends of a plate-shaped piezoelectric body, and a center region sandwiched between regions of both the right and left ends is not provided with an electrode to be set as a region to determine a resonant frequency, and groove-shaped cuts are provided between the region of the right end and the center region and between the region of the left end and the center region, thereby separating the respective regions elastically and moderately. Further, in Patent Document 2 and Non Patent Document 1, there is disclosed a piezoelectric vibrator in which electrodes for excitation are provided on regions of both right and left ends of a plate-shaped piezoelectric substrate, and an electrode is not provided on a center region sandwiched between these regions, and by making a thickness of the region having no electrode provided thereon thicker than those of the regions of both the ends, resonant frequencies of these regions are made approximately the same, thereby enabling the respective regions to be coupled to one another acoustically.
Besides the above, in Non Patent Document 2 and Non Patent Document 3, there is disclosed a resonator of a type in which a region without an electrode, which is formed thicker than a periphery and called a mesa, is provided at a center portion of a plate-shaped piezoelectric body, and electrodes for excitation are formed in a groove formed in a periphery of this mesa to obtain resonance by using the electrodes and a trapping energy effect to occur in the mesa surrounded by the electrodes.
These resonators (the piezoelectric vibrator) disclosed in Patent Document 1, Patent Document 2, and Non Patent Document 1 to Non Patent Document 3, by providing the region of a center side without an electrode being provided independently of the regions for excitation provided with the electrodes, manage to have little effect of the change with time to occur in the electrodes on the region of the center side, and thus increase long-term frequency stability as a whole of the resonator or the piezoelectric vibrator (the elastic wave device).
Incidentally, in Patent Document 3, there is disclosed a technique in which thicknesses of regions of both ends with electrodes provided thereon of a vibration wave propagating medium to be a vibrator are thinned, and on the other hand, a thickness of a center region sandwiched between these regions and having no electrode provided thereon is thickened, and further a length of the above center region is adjusted to satisfy a predetermined condition, thereby taking out only a desired harmonic vibration from among a plurality of harmonics excited in the electrode on one side. As described above, in the vibrator disclosed in Patent Document 3, an object of providing the center region without an electrode is different from that of stability of a resonant frequency.
[Patent Document 1]
    Japanese Patent Publication No. Sho 43-15538: page 2 left column lines 12 to 23, FIG. 2a[Patent Document 2]    Japanese Patent Application Laid-open No. 2000-91878: claim 1, paragraphs 0024 and 0025, FIG. 4[Patent Document 3]    Japanese Patent Publication No. Sho 50-19024: page 173 column 2 lines 16 to 30, FIG. 2(b)[Non Patent Document 1]    “IMPROVEMENT OF FREQUENCY DRIFT OF AT-CUT RESONATORS USING AN UNELECTRODED RESONANT REGION”, IEEE International Frequency Control Symposium and PDA Exhibition, 2001, p. 624, FIG. 1(b), FIG. 7[Non Patent Document 2]    “STRAIGHT CRESTED WAVE ANALYSIS OF QUARTZ MEMS RING ELECTRODED MESA RESONATORS”, IEEE ULTRASONICS SYMPOSIUM, 2002, P. 999, FIG. 2[Non Patent Document 3]    “3-D MODELING OF HIGH-Q QUARTZ RESONATORS FOR VHF-UHF APPLICATIONS”, Proceedings of the 2003 IEEE International Frequency Control Symposium and PDA Exhibition Jointly with 17th European Frequency and Time Forum, 2003, p. 824, FIG. 2